1. Filed of the Invention
The invention relates to a method for controlling the final thickness of a rolled product, at the outlet of a tandem rolling mill, enabling in particular to optimise the productivity of such a plant while balancing the currents of the driving motors of the different stands, in order to enable an increase in the overall rolling speed, without any risks of overloading either of the motors. The invention also relates to a control device enabling the implementation of such a method.
The invention is provided especially for cold rolling of metal bands, for instance of steel, but may be applied, generally, to any plant including several roll stands operating in tandem for gradual reduction in thickness of a product running successively between the working rolls of said stands.
2. Brief Description of Related Art
It is known that a rolling mill includes, generally, at least two working rolls mounted inside a supporting stand and delineating a gap for letting through the product to be rolled, the stand carrying means for applying an adjustable clamping load between the rolls. The number of rolls may vary according to the type of rolling mill for instance duo, quarto, sexto or other.
To determine the infeed of the product between the rolls, the latter are driven into rotation around their axis by motorised means which apply a driving torque, either directly to the working rolls, or indirectly, to the back-up rolls in a quarto assembly or to intermediate rolls in a sexto assembly.
For a long time so-called <<in tandem>> rolling plants have been known, including at least two successive stands each performing a portion of the reduction in thickness. From a raw thickness, the product is therefore subjected, in the first stand, to a first reduction in thickness and it comes out at a speed determined by the rotational speed of the working rolls. In the second stand, it is subjected to a second reduction in thickness and comes out at a greater speed to follow to the mass flow preservation law. The working rolls of the second stand must therefore be driven into rotation at a speed greater than that the rolls of the first stand, these speeds being in the reverse proportion of the reductions performed in each stand.
Besides, the rotational torques applied to the working rolls are adjusted so that each intermediate stand exerts a traction load on the band coming out of the previous stand.
It is necessary to ensure control, on the one hand, of the reduction in thickness performed in each of the stands in order to obtain, at the outlet of the plant, a product having constant thickness with a certain degree of accuracy and, on the other hand, to keep the band perfectly stretched in each so-called <<inter-stand>> space between two successive stands, in order not to reach the traction levels which might cause the band to break.
Usually, the thickness of the band running through the successive stands of a tandem rolling mill is controlled by monitoring the mass flow ratio.
In a known control method, used conventionally to obtain, at the outlet of the plant, a band with a given thickness, the thickness of the band at the outlet of the first stand is kept constant, on the one hand, and the speed ratios between the first and the last stand are held constant, on the other hand.
The speeds of the intermediate stands may be deducted from these conditions since they are imposed by the mass flow preservation law of the metal running through the stands of the rolling mill, and they are reversely proportional to the reductions ascribed to each rolling stand.
The thickness at the outlet of the first stand is generally controlled, on a modern rolling mill, by the clamping means which are driven by a feeler gauge situated downstream of said stand. Certain systems, more sophisticated, also include a feeler gauge upstream of said stand.
The whole control system of a tandem rolling mill is currently called <<automatic gage control>> or AGC.
Besides, in order to regulate the traction loads in the inter-stand spaces, one acts generally on the clamping means of the stands, since it is not possible to modify the speed ratios between the successive stands without affecting the outlet thickness. To do so, each inter-stand space receives a traction measuring device such as a tensimeter roll which controls the clamping level of the stand situated downstream. A feeler gauge, placed at the outlet of the rolling plant, controls the final thickness by acting on the speed of the last stand or of the last two stands of the tandem rolling mill. Such a system for controlling the inter-stand tractions is also called <<automatic tension control>> or ATC.
In each stand, the strength and the rolling torque applied, respectively, for a certain reduction in thickness, by the clamping means and by the driving means of the working rolls, should be suited to the characteristics of the product to be rolled. For each type of product, a <<rolling pattern>> should therefore be worked out, which determines the successive reductions in thickness allocated to each stand relative to geometric and metallurgic characteristics of the product.
However, it is not possible to ask the operators to establish, optimally and permanently, a rolling pattern for each product involved in the annual production of the rolling mill.
As generally known, to obtain such a result automatically, a pre-adjustment system may be used for calculating the rolling patterns, considering all the characteristics of the plant such as the powers of the driving motors, the maximum intensities and speeds of the motors, the possible maximum stresses on the roll stands, etc. This pre-adjustment system must also take into account the geometric and metallurgic characteristics of the product to be rolled and the product/rolling mill interface to establish the rolling parameters adapted to each format and nature of band forming the annual production of the rolling mill. These parameters are, in particular, the inlet thickness and the outlet thickness, possibly the temperature, the hardness, or still the flow constraint and the variation of this constraint over the reduction in thickness, as well as the friction coefficient in the sheet/roll interface.
This pre-adjustment system may be in the form of multiple inlet tables providing with the adjustments to be displayed for each stand relative to the inlet parameters. In certain systems known, the operators input beforehand the characteristics of the bands to be rolled according to the programme of production forecast and it then suffices to validate such data at the arrival of the head of the band of the product considered in the rolling plant.
However, it is also possible to use more sophisticated pre-adjustment systems including a mathematical model which calculates a reduction pattern for each band entering the rolling mill tandem. Such a model then establishes possible reduction values for the stands and may perform certain optimisations in order to choose the rolling pattern corresponding to the best power distribution. The more sophisticated models may also be reset by frequently recording the actual values of the rolling parameters such as the rolling stresses, the torques applied by the motors and their speeds.
Moreover, it must also be possible to vary the overall speed of the rolling plant in order to accelerate or to slow down the product at the outlet of the plant. Still, the mass preservation law only enables to adjust the speeds with respect to one another, as a relative value. In a known process, one acts therefore on the speed of one of the stands, called a pivoting stand and the speed of the other stands is managed by a system of controls in order to keep the speed ratios corresponding to the distribution of the reduction rate between the different stands.
In practice, the means for driving the rolls into rotation are electric motors with a basic speed for which they provide their rated torque. Consequently, when designing the rolling mill train, an average reduction in thickness is considered for each stand. The motors being, generally, built to have the same basic speed, a speed reducer is installed very often between the motor and the stand, whereof the reduction ratio is different for each stand in order to obtain the same speed on the high speed shaft of the reducing gear.
This overall design of the tandem rolling mill with a speed gradation on the high speed shaft, determining the rotational speed of the milling rolls, from the first stand to the last, is called commonly <<speed cone>>.
Still, during actual production, the exact reduction ratio to be applied to each stand in order to obtain on the product the reduction in thickness desired, does not coincide perfectly with the speed gradation of the motors. There results that all the motors do not lie on the same operating point. To increase the overall rolling speed, certain motors will therefore reach their intensity limit before others and then prevent a production at optimum speed of the plant.
Consequently, in a very large of number of cases, the maximum speed possible may not be reached and the productivity of the rolling plant does not correspond to its maximum capacity.
The pre-adjustment systems used currently do not enable to solve this problem. Indeed, certain important rolling parameters such as the friction coefficient between the band and the milling rolls, which depends on the surface conditions and on lubrication, are accessible to the adjustment patterns only by very indirect calculation on the basis of the intensity, the strength and the speed measured. When changing the working rolls, the diameter and the surface condition of the rolls will therefore change, as well as the thermal equilibrium of the rolling mill. Even if a mathematic model has been used, said model will not find very rapidly correct adjustment of the reductions per stand enabling to obtain the maximum speed of the plant, therefore its optimum productivity.